The broken barrel of a maple bat whacked fan Susan Rhodes, 50, in the head as she sat four rows behind the visitors' dugout at Dodger Stadium in Los Angeles on April 25. She didn't see it coming. She suffered a concussion and the blow fractured her jaw in two places.

Broken bats are commonplace in baseball games, but the Rhodes incident along with similar injuries this year to a hitting coach and an umpire, are making people wonder: Has America's pastime suddenly become a lot more dangerous and is the new trend in bat wood to blame?

Babe Ruth's hickory bats are long gone, and now it seems the decades-long tradition of ash bats might also be waning. Thanks to Barry Bonds' affinity for maple bats, more and more players are using maple and an argument can be made that they are more likely to break.

"It's really dangerous," Atlanta Braves manager Bobby Cox told The Atlanta Journal-Constitution on June 26, referring to the prevalence of breaking bats. Just the week before, he was watching the ball when a piece of second baseman Kelly Johnson's bat flew over his head. Like Rhodes, Cox never even saw it.

Last month, players, teams and league officials began meeting to decide what might be done to solve the broken bat problem. Scientists and engineers have also considered the problem — they know that differences between maple bats and the more traditional ash bats, as well as the ways that a bat is shaped and hit, can influence how and whether it breaks.

Bat evolution

The maple vs. ash controversy didn't exist in the early days of baseball: When Babe Ruth was hitting homers, he — and every other player — used a hickory bat.

"Hickory was a common wood, and it's still known today as a good strong wood," said Lloyd Smith, a mechanical and materials engineer from Washington State University. "But it is very heavy ... that was one of the criticisms, was that it was a heavy bat."

The desire for a lighter-weight bat (for faster swinging and higher batting averages) eventually led to the adoption of ash as the wood of choice for major leaguers. And it stayed the preferred type of wood up until a few years ago.

But because it is lighter, ash is not as strong hickory.

"The problem with most wood is that strength is proportional to weight, so if you want a really strong wood, you can do that, but you end up getting an increase in weight," Smith explained. "And if you want a really light wood, you can do that, but you pay for it because your strength goes down. So there's this kind of optimum balance."

In the 1990s, maple started to make the rounds as an alternative. It was appealing because it was stronger (which is better for hitting longer distances) and less prone to flaking than ash, so players didn't go through bats as quickly. Most players still stuck to their ash bats, though — that is, until Barry Bonds got the single-season home run record in 2001, using a maple bat.

Now, just a few years later, maple is no longer on the fringe.

"For 50 years, northern white ash was the wood. Today half of the bats in the major leagues are made out of maple. So it was a very dramatic shift," Smith told LiveScience.Flaking, cracking and breaking

Maple and ash tend to break in different ways. While ash tends to crack and flake off in smaller chunks, maple tends to fracture in bigger, jagged shards.

Smith attributes some of the difference in breaking patterns to the structure of the pores, which transport moisture inside the trees before they become bats.

Ash is what is called ring porous. "If you were to kind of climb inside of the wood, what you find is, in the grain areas, there's a whole bunch of pores that carry moisture through the tree. And if you go in the region of the growth ring where you don't have the grain, it's more or less solid fiber," Smith said.

Because the voids in the wood are concentrated in a few areas, the growth planes have weak regions. When the ash bat hits a ball, "these cell walls would collapse, and you'd get what they call 'flaking' — the barrel would just kind of start to soften, and you'd get little layers that flake off," Smith said.

Maple on the other hand is "ring diffuse," meaning its pores are more evenly distributed throughout the wood.

"So a characteristic of maple that exists today is the barrel is very durable; you don't get these flaking kinds of failures in maple bats that you did in the ash bats," Smith said.

Cracking styles

Cracks form in both types of wood as a bat is used to hit ball after ball after ball. But the same pore structure that makes ash prone to flaking also channels cracks along the length of the bat, meaning the crack has a long way to grow before it can break the bat in two. And batters tend to notice the cracks or decide the bat has too much flaking and switch to a new bat before the old bat completely breaks.

Because of maple's diffuse pores, cracks in the wood can grow in any direction, making it easier for them to grow out toward the edge of the barrel, causing a large chunk of it to break off entirely. And since maple doesn't flake, serving as a warning to a player that his bat is cracking, "you're perhaps more likely to have bat particles flying through the infield," Smith said.

How the bat is cut out of the wood when it is made can affect its susceptibility to breakage as well. A bat is strongest when the grain lines up with the length of the bat. The grain of ash is easier to see and straighter than the grain of maple, which Smith says could be a factor in how and how often maple bats break.

"If you have a bat that's not cut straight to the grain, it's going to be a weaker bat," Smith said. "Now whether that's the cause of the maple failures or not, there's still other things that could be going on, but that could at least be one factor."

Tremendous force

Another such factor has to do more with the batter than the wood: He could hit the ball badly.

The ball comes into contact with the bat over a small area for only about one thousandth of a second; the force of that short impact is about 5,000 pounds.

"If you hit the ball poorly, if you don't hit it on what they call the 'sweet spot' of the bat, you get this kind of stinging feeling in your hands," which means the bat is vibrating and bending, Smith explained.

If the vibrations are large enough, the bending can cause the bat to break, usually at the narrowest part of the bat, the handle. (That's exactly where the bat of Colorado Rockies player Todd Helton broke before the barrel hit Rhodes, the fan.)

This leads to another aspect of today's bats that could be causing them to break: narrow handles.

A century ago, bat handles were much thicker than they are today. Smith attributes the narrowing of the handle to the advent of metal bats, which most players today grew up using, and which typically have narrower handles. A narrow handle makes a wooden bat less sturdy and more prone to breaking.

Controversy today

The seeming prevalence of breaking bats in games this season has brought the issue into the limelight. The most recent incident occurred when plate umpire Brian O'Nora was hit in the head by a stray bat shard in a game between the Colorado Rockies and the Kansas City Royals on June 24.

But though engineers like Smith have a good idea of how and why bats crack and break, there is little data on how often they do so and which types of wood break more often, so there's no real evidence that bats are breaking more often now than in the past, or that maple breaks more than ash.

"People are really focusing on maple because it perhaps has a more dramatic failure than ash," Smith said, adding that broken ash bats have also caused injuries in the past.

One factor that Smith suggests could be skewing the statistics on which type of bat breaks more often is the fact that cracks in ash bats can often be detected before the bat breaks (players tap the bat on the plate and can tell that it sounds different), whereas maple cracks usually can't be detected and are more likely to break during a swing.

A number of ways to reduce the number of broken bats have been suggested. Smith mentioned the simple option of requiring thicker handles, like the older baseball bats (there are currently no restrictions on handle diameter in the major leagues).

"If you increase the handle diameter, then you're going to make the bat stronger, no question," Smith said. But that alone won't solve the problem.

"Really the problem is part of the game," Smith added. "Wood bats fail, and they'll continue to fail, and maple bats will likely continue to fail in a more brittle way than ash bats."

Major League Baseball could also do a study of wood types and put restrictions on the species that break in a more brittle way, or put specifications on the grain alignment of the bats to make them less likely to break, Smith said.

Alternatively, more protective netting could be added in front of the lower-level infield seats in stadiums, which would protect fans (Detroit Tigers center fielder Curtis Granderson suggested this option in his ESPN.com blog, since fans are the primary concern for injuries because it is easier for players to dodge errant shards). Smith agrees that it would keep fans safer, but adds: "Then you've got to look through that stupid net to enjoy the game."

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